The field of the invention is that of protecting active imaging systems against stray reflections and/or scattering and against laser attacks.
Active laser imaging is an optronic technique for obtaining images of a scene under laser illumination. An active imaging system comprises:                a laser source with ocular safety for illuminating the target and the neighbouring scene,        a mono-detector or a matricial detector sensitive in the operating band of the laser, adapted to detect the beam back-scattered by the illuminated scene,        the optics and the signal processing necessary for constructing the image of the scene.        
The application fields of active imaging are particularly important, notably for defence, security and biomedicine.
Beyond conventional bidimensional imaging, it is possible to acquire mono-dimensional signatures with a very short-pulse laser having ocular safety. These signatures make it possible to identify the object by virtue of the back-reflection of the pulse from the various parts of the object. A very high-resolution signature is thus obtained, allowing the profile of the object to be determined precisely. By using an angular scan of the signature, it is then possible to reconstruct a three-dimensional tomographic image of the objects. These techniques are also used in reflection to reconstruct biological tissue.
Identification by profilometric images, either direct bidimensional or tridimensional for tomographic reconstruction, is possible only if the signal-to-noise ratio received by the detection system is sufficiently high and if the noise sources are greatly reduced.
One particularly important noise source in active systems corresponds to auto-glare of the detector by stray reflections and/or scattering from the lenses of the optical device or from atmospheric layers.
The lasers employed in active imaging systems have a high pulse energy in order to obtain large identification ranges. Any reflection/scattering from the lenses of the emission optics or from the first atmospheric layers therefore increases the risk of dazzling the sensor. Time windowing, or “gating”, partially makes it possible to deal with auto-glare of the detectors, but on certain detectors there is a remanence which leads to dazzling of the detection matrix. Furthermore, no civil component makes it possible to fulfil this function in view of the energies and pulse durations which are different from those of telecommunication applications. The techniques of electronic “gating” remain complex to implement, notably for durations less than about ten nanoseconds, and they do not make it possible to deal with destruction of the sensitive parts of the detectors. Eliminating these stray reflections ensures optimal functioning of the system.
In order to overcome atmospheric conditions and improve the resolution in relation to 3-5 μm or 8-12 μm infrared band passive imaging, active imaging systems have been developed in the near IR and particularly in the spectral range centred on 1.5 μm, which makes it possible to construct systems with ocular safety.
In the field of security and defence, these imaging systems can be dazzled or destroyed by a high-power laser source in the same band. The sensitivity of their detection system, and their use at limiting range can expose them to relatively effective dazzling. Specifically, work on lasers emitting around 1.5 μm has led to an increase in the available energy by using quasi-phase-matched materials. In view of their reduced spectral operating width, active imaging systems have increased vulnerability to attacks by these lasers. It is therefore necessary to equip imaging systems with protection of their detection device against these laser attacks.
The devices for active limitation of the transmitted fluxes therefore have twofold interest for active imaging systems at 1.5 μm.
In order to produce such a device, the protective function should be reversible, that is to say the device should become transparent again after a laser attack. It should therefore have a good flux resistance, possibly be insensitive to polarization, should respond for pulses with a duration of a few ns and have an extended protective dynamic range.
Protection devices exist; they use mobile optical filters, optical configurations with separate paths in emission and reception, and specific designs of the readout circuits of the detectors. All these devices have significant drawbacks, notably in terms of:                response times,        complexity of the optical systems in terms of cost and size,        the electronics in terms of development cost.        
Other devices have been proposed, which use materials with nonlinear absorption.
Among the various types of nonlinear absorption, two-photon absorption or TPA may be mentioned. This is a process illustrated in FIG. 1, during which two photons are absorbed simultaneously by a material. The molecules change by two-photon absorption from the ground level S0 to an allowed excited level S1, the energy of which corresponds to the sum of the energies of the two incident photons. When the population of the excited state S1 becomes significant, it may be necessary to take into account “linear” absorption from the excited state, which makes the molecule change from the state S1 to the state S2.
In this type of material, when the pulse duration is wide compared with the lifetime of the excited state, the transmission of a material with a length L may be calculated from the following propagation equation:
            ⅆ      I              ⅆ      z        =                    -                  α                      (            1            )                              ⁢      I        -                  α                  (          2          )                    ⁢              I        2              -                  α                  (          3          )                    ⁢              I        3            
where I is the optical illumination (in W m−2)
z is the propagation coordinate through the nonlinear material,
α(1) is the linear absorption coefficient
α(2) is the two-photon absorption coefficient
α(3) is the apparent three-photon absorption coefficient which is due to linear absorption from the excited state (absorption following a first step of two-photon absorption). This coefficient is linked with the effective absorption cross section from the excited state σ12:
      α          (      3      )        =                    α                  (          2          )                    ⁢              σ        12            ⁢              τ        1              hv  
where τ1 denotes the lifetime of the excited state S1, and τ2 denotes that of the excited state S2,
hν is the energy of the photon.
In the figure, the effective two-photon absorption cross section from the ground state is denoted σ01, and the lifetime of the excited state S2 is denoted τ2.
The materials having nonlinear absorption which are used in the visible band only make it possible to cover a reduced spectral range (typically for a wavelength of between 400 nm and 800 nm), and are not therefore effective up to 1.5 μm. For systems operating around 1.5 μm, the active material may be a semiconductor such as for example AsGa, although its nonlinear TPA is strongly dependent on traps (impurities).